Ethanol for industrial use is conventionally produced from petrochemical feed stocks, such as oil, natural gas, or coal, from feed stock intermediates, such as syngas, or from starchy materials or cellulose materials, such as biofuels. Conventional methods for producing ethanol from petrochemical feed stocks, as well as from cellulose materials, include the acid-catalyzed hydration of ethylene, methanol homologation, direct alcohol synthesis, and Fischer-Tropsch synthesis. Instability in petrochemical feed stock prices contributes to fluctuations in the cost of conventionally produced ethanol, making the need for alternative sources of ethanol production all the greater when feed stock prices rise. Starchy materials, as well as cellulosic materials, are converted to ethanol by fermentation. However, fermentation is typically used for consumer production of ethanol. In addition, fermentation of starchy or cellulosic materials competes with food sources and places restraints on the amount of ethanol that can be produced for industrial use.
Ethanol production via the catalytic hydrogenation of alkanoic acids and/or other carbonyl group-containing compounds, such as acetic acid, has been widely studied, and a variety of combinations of catalysts, supports, and operating conditions have been mentioned in the literature. Excess of hydrogen is used to increase the yield of ethanol production and hence it is beneficial to recycle the unreacted hydrogen back to the reactor. However, during the hydrogenation of acetic acid, other compounds are formed with ethanol or are formed in side reactions. These impurities can limit the production and recovery of ethanol from the reaction and/or have a deleterious effect on the life or selectivity of the catalyst. One such impurity is carbon monoxide which, if not removed, can build up in the hydrogen recycle stream.
EP2060555 describes a process for the production of ethanol from a carbonaceous feedstock; wherein the carbonaceous feedstock is first converted to synthesis gas and the synthesis gas is then converted to ethanoic acid, which is esterified and then hydrogenated to produce ethanol. Part of the hydrogen recycle stream is purged to control the build-up of diluents in the hydrogenation reactor. However, purging the hydrogen recycle stream results in the loss of reactants and reduces operating efficiencies.
There is therefore a need for an improved process for reducing the level of carbon monoxide in the process gas streams employed in the hydrogenation of acetic acid to produce ethanol.